Acoustic therapies include shock wave lithotripsy (SWL), high intensity focused ultrasound (HIFU), and ultrasound-enhanced drug delivery. HIFU is used for many therapeutic applications, including hemostasis, tumor treatment, and tissue necrosis. These procedures are made possible by the unique ability of such acoustic therapy technologies to selectively apply relatively large amounts of therapeutic energy (on the order of 1000 W/cm2) to a treatment volume disposed deep within a body mass, without adversely affecting tissue disposed between an acoustic therapy transducer that produces the energy and the treatment volume. HIFU, in particular, is a powerful medical technique with great potential and is currently being employed, both in the United States and abroad, to treat tumors. However, to safely implement noninvasive, HIFU-based transcutaneous acoustic surgery, a medical imaging modality must be used to visualize the internal treatment site, for targeting the site and monitoring the treatment process. Ultrasound imaging is an attractive modality for the following reasons: (a) images are available in real-time; (b) portable imagers are commercially available; (c) Doppler-based imaging modalities can be used to detect bleeding; (d) ultrasound imaging is a relatively ubiquitous medical technology; and, (e) ultrasound imaging is relatively inexpensive, compared to other medical imaging systems, such as magnetic resonance imaging (MRI).
A problem with combining HIFU therapy with ultrasound imaging is that the high energy therapeutic waves introduces a significant amount of noise into an ultrasound imaging signal employed to monitor the treatment site, making simultaneous imaging and treatment difficult. Indeed, the high energy of the HIFU wave can completely overwhelm conventional ultrasonic imaging systems. One analogy that might help to make this problem clear relates to relative intensities of light. Consider the light coming from a star in the evening sky to be analogous to the low power imaging ultrasound waves that are reflected from a target area toward the imaging transducer, while the light from the sun is analogous to the HIFU waves generated by the therapy transducer. When the sun is out, the light from the stars is completely overwhelmed by the light from the sun, and a person looking into the sky is unable to see any stars, because the bright light from the sun completely masks the dim light coming from the stars. Similarly, the HIFU waves emitted by the therapy transducer completely overwhelm the lower energy imaging ultrasound waves produced by the imaging transducer, and any ultrasonic image generated is saturated with noise caused by the HIFU wave from the therapeutic transducer.
FIG. 1A schematically illustrates a prior art ultrasound image 10 in which a scanned field 12 is completely obscured by noise 14, caused by the simultaneous operation of an ultrasound imaging pulse (i.e., an ultrasound imaging wave) and a HIFU wave (neither shown). In ultrasound image 10, a clinician may be attempting to focus the HIFU wave on a treatment site 18. However, because noise 14 completely saturates scanned field 12, it is impossible to accurately focus the HIFU wave onto treatment site 18. If the therapy transducer is completely de-energized, noise 14 is eliminated from the scanned field. However, under these conditions, the focal point of the HIFU wave will not be seen, and thus, the HIFU wave cannot be accurately focused on treatment site 18. While some change in echogenicity at the HIFU focal point may persist for a time even after the HIFU wave is no longer active, any change in a position of the therapy transducer (or treatment site 18) will not register until the therapeutic transducer is re-energized. Thus, the HIFU wave cannot be focused in real time.
Some prior art systems have included a targeting icon in an ultrasound image to indicate where the known focal point of a specific HIFU transducer would be located in a scanned image. While this icon may be helpful in determining a position of the focal region of the HIFU transducer relative to the scanned ultrasound image, such an icon based technique does not enable a clinician to observe real-time results. Once the HIFU therapeutic transducer is energized, the scanned ultrasound image is completely saturated with noise, and the clinician cannot monitor the progress of the treatment without again de-energizing the HIFU therapeutic transducer. Furthermore, it should be noted that the accuracy of such icon-based targeting systems generally degrades during treatment due to changes in refraction, temperature of the tissue, the presence bubbles in or near the target area, and patient movement (including movement associated with respiration).
FIG. 1B schematically illustrates a prior art technique disclosed in U.S. Pat. No. 6,425,867 (the disclosure, specification and drawings of which are hereby specifically incorporated by reference) for reducing the amount of noise disrupting an ultrasound image during HIFU therapy. In FIG. 1B, the HIFU wave generated by the therapeutic transducer has been pulsed. This technique produces an ultrasound image 20, in which the location of noise 24 in a scanned field 22 is a function of the interference between the pulsed HIFU wave generated by the therapy transducer and the ultrasonic imaging pulses generated by the scanning transducer. In FIG. 1B, noise 24 substantially masks a treatment site 28. This result would not occur in all cases, because to an observer, noise 24 would move across scanned field 22 as the interference between the HIFU waves and the imaging pulses varies in time. Pulsing of the HIFU wave alone would thus enable the clinician to view a noise-free image of the treatment site only when noise 24 was randomly shifted to a different part of scanned field 22, away from the treatment site. However, such pulsing alone generates an image that is extremely distracting to a clinician, because noise 24 flickers across scanned field 22, making it difficult to concentrate and difficult to consistently determine where the focal point of the HIFU wave is, relative to the treatment site, in real time.
FIG. 1C schematically illustrates another prior art technique that is disclosed in U.S. Pat. No. 6,425,867 (referred to hereafter as the '867 patent), also for reducing the amount of noise disrupting an ultrasound image during HIFU therapy. In an ultrasound image 30, a HIFU wave from a therapy transducer has been both pulsed and synchronized with respect to the ultrasonic imaging pulses from an imaging transducer, to ensure that noise 34 does not obscure a treatment site 38. In ultrasound image 30, noise 34 has been shifted to a location within a scanned field 32 that is spaced apart from treatment site 38, by selectively adjusting both the pulsing and the synchronization of the HIFU wave. Preferably, noise 34 is shifted completely away from treatment site 38, thus providing the clinician a noise-free, stable image of treatment site 38 that clearly shows the location of the focal point of the HIFU wave relative to the treatment site. Thus, the HIFU wave can be focused onto treatment site 38, in real time. By synchronizing the HIFU bursts within each imaging frame, the interference can be relegated to certain portions of the image, such as a fringe of the ultrasound image, enabling other portions of the ultrasound image to remain useful for monitoring and guidance. If the imaging process and the HIFU bursts are not synchronized, the interference will randomly obscure the treatment site, as indicated in FIG. 1B.
FIG. 2 is a block diagram from the '867 patent, schematically illustrating a system that synchronizes the ultrasound image and HIFU waves required for the simultaneous imaging and therapy in real time. A conventional imaging probe 44 is connected to an ultrasound imaging machine 40 via a cable 42. Imaging probe 44 generates ultrasonic imaging pulses that propagate to the target area, are reflected from structure and tissue within the body, and are received by the imaging probe. The signal produced by the imaging probe in response to the reflected ultrasound imaging waves is communicated to the ultrasound imaging machine through cable 42 and processed to provide a visual representation of the structure and tissue that reflected the ultrasonic imaging pulses. An imaging beam sector 46 (indicated by dotted lines) from imaging probe 44 is identified in the Figure by dash lines. The system described in the '867 patent also includes a therapeutic transducer 60. When excited, this therapeutic transducer generates HIFU waves that are focused at a particular point of interest, i.e., a treatment site within a patient's body. In FIG. 2, the path of a HIFU beam 62 (indicated by solid lines) narrows to a focal point 64.
Synchronization output signal 48 is supplied to a synchronization delay 50, which enables the user to selectively vary the initiation of each HIFU wave with respect to each sequence of ultrasonic imaging pulses that are generated to form an ultrasonic image. Referring to FIG. 1C, delay 50 enables a user to vary the position of noise 34 in scanned field 32, so that the noise is moved away from treatment site 38, to a different portion of scanned field 32. A HIFU duration circuit 52 is used to control the duration of the HIFU wave. A longer duration HIFU wave will apply more energy to the treatment site. If the HIFU wave is too long, the duration of noise 34 as shown in ultrasound image 30 will increase and can extend into the next ultrasound imaging pulse to obscure treatment site 28, or may completely obscure ultrasound image 30, generating a display very similar to ultrasound image 10 in FIG. 1A. Thus, the user will have to selectively (i.e., manually) adjust HIFU duration circuit 52 to obtain a noise-free image of treatment site 38, while providing a sufficient level of energy to the treatment site to affect the desired therapeutic effect in an acceptable time. A HIFU excitation frequency generator 56 is used to generate the desired frequency for the HIFU wave, and a power amplifier 58 is used to amplify the signal produced by the HIFU excitation frequency generator to achieve the desired energy level of the HIFU wave. Power amplifier 58 is thus adjustable to obtain a desired energy level for the HIFU wave.
Significantly, the system disclosed in the '867 patent requires modifying a conventional ultrasound imaging machine to achieve modified ultrasound imaging machine 40, which is capable of providing synchronization output signal 48. The '867 patent notes that such a synchronization output signal is not normally provided in prior art ultrasound imaging machines. The '867 patent suggests that if an ultrasound imaging machine capable of providing the synchronization output signal is not available, then a synchronization output signal can be derived from the ultrasound imaging signals conveyed by cable 42. The '867 patent also suggests that an optional stable synchronization signal generator 66 can be used to synchronize the HIFU wave to the imaging ultrasonic wave, instead of using synchronization output signal 48 from ultrasound imaging machine 40. Stable synchronization signal generator 66 can be used to provide a stable synchronizing pulse to initiate the HIFU wave, and the timing of this stable synchronizing pulse can be manually varied until a noise-free image of the treatment site has been obtained. A drawback of using stable synchronization signal generator 66 instead of synchronization output signal 48 is that any change in the timing of the ultrasound imaging pulses, such as is required to scan deeper within tissue, will require to the user to again adjust stable synchronization signal generator 66. Such an adjustment would not be required if synchronization output signal 48 were used. It should be noted that one drawback of using synchronization output signal 48 is that the ultrasound imaging system must be modified or custom built to provide such a synchronization signal. Furthermore, some imaging modalities, such as Doppler imaging, have very complex signals, and synchronization output signal 48 may not be very effective for synchronizing such complex signals.
Essentially, the '867 patent addresses HIFU interference of ultrasound imaging by synchronizing the interference so that the interference is stable and is located at the fringes of the image. As a result, the region of interest in the image is not obscured (as is schematically indicated in FIG. 1C). This functionality requires knowledge of the frame rate and phase of the imaging cycle, both of which vary with changes to user control settings (particularly depth and switching modality from b-mode to Doppler). Once the frame rate and phase are known, HIFU can be gated synchronously with the imaging cycle and the interference that is caused can be moved to the fringes of the image. Unfortunately, there is no simple way of determining the frame rate and phase of a stand-alone commercial imager that has not been designed to provide such information (i.e., which has not been modified to provide synchronization output signal 48).
As indicated in the '867 patent, ultrasound imaging systems can be designed to incorporate a synchronization output signal. However, even though ultrasound imaging systems are significantly less expensive than MRI imaging systems, high end ultrasound imaging systems can still cost in excess of $150,000, and it would be desirable to provide a synchronization technique that is compatible with ultrasound imaging systems that do not provide a synchronization output signal (the majority of ultrasound imaging systems sold do not support the synchronization output signal as described in the '867 patent). The '867 patent also suggests that the synchronization signal (frame rate without phase information) could be obtained from the cable coupling an ultrasound imaging probe to ultrasound imaging machines. This theoretically could be achieved by detecting current in the cable. However, such cables include many wires and currents, and such cables are well shielded to meet safety standards. Hence, obtaining the signal necessary for synchronization from a shielded cable is challenging. The cable could be modified to facilitate extraction of the synchronization signal; however, that modification is not likely to be supported by the manufacturers of the ultrasound imaging equipment, and operators of medical equipment are not likely to pursue a modification not sanctioned by a manufacturer, particularly because of liability and warranty concerns. Thus, it would be desirable to provide a technique for synchronizing HIFU interference in an ultrasound image, without requiring the use of a stable synchronization signal generator as disclosed in the '867 patent. The synchronization should also be achieved without modifying an ultrasound imaging apparatus to provide a synchronization signal.